Driveline components, such as gears, for example, are traditionally formed from a low carbon content steel. One example of a gear material is SAE 8822H, which is a carburizing grade alloy steel. SAE 8822H has the following chemical composition, in combination, by weight: 0.19-0.25% carbon (C), 0.70-1.05% manganese (Mn), 0.15-0.35% silicon (Si), 0.35-0.75% nickel (Ni), 0.35-0.65% chromium (Cr), 0.30-0.40% molybdenum (Mo), no more than 0.035% phosphorous (P), and no more than 0.040% sulfur (S), with the balance being essentially iron (Fe).
Some gear steels, such as SAE 8822H, are specially designed carburization grade steels that are alloyed-low carbon content steels (0.10-0.27% carbon), which traditionally are expensive. Carburizing is a process in which carbon is added to a surface of an iron-base alloy by absorption through heating the alloy at a temperature below a melting point of the alloy, while providing contact with carbonaceous solids, liquids, or gases. In order to achieve desired final hardness and surface characteristics, the SAE 8822H material is carburized, quenched, and tempered.
Carburization is a prolonged process and can take as long as ten to twenty-four hours, depending on case depth requirements. Prolonged processing and expensive steel grades increase manufacturing costs for gears and other driveline components. Also, the prolonged carburization process causes non-martensite transformation products (NMTP) and intergranular oxides (IGO) to form at a surface of the component. NMTP and IGO adversely affect bending fatigue strength and wear resistance. Thus, the occurrence of both NMTP and IGO can significantly reduce service life of the component.
High carbon content steels (0.60-0.80% carbon) can also be used to form driveline components. Some examples of high carbon content steels are disclosed in RU2158320. These examples include 62ΠΠ1, 62ΠΠ2, 62ΠΠ3, 62ΠΠ4, 62ΠH1, and 80ΠΠ1.
62ΠΠ1 has the following chemical composition, in combination, by weight: 0.60-0.67% carbon (C), 0.05-0.15% manganese (Mn), no more than 0.05% silicon (Si), no more than 0.10% chromium (Cr), no more than 0.10% nickel (Ni), no more than 0.10% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), no more than 0.40% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
62ΠΠ2 has the following chemical composition, in combination, by weight: 0.60-0.67% carbon (C), no more than 0.10% manganese (Mn), 0.10-0.20% silicon (Si), no more than 0.10% chromium (Cr), no more than 0.10% nickel (Ni), no more than 0.10% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), no more than 0.40% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
62ΠΠ3 has the following chemical composition, in combination, by weight: 0.60-0.67% carbon (C), 0.05-0.15% manganese (Mn), 0.05-0.15% silicon (Si), no more than 0.10% chromium (Cr), no more than 0.10% nickel (Ni), no more than 0.10% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), no more than 0.40% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
62ΠΠ4 has the following chemical composition, in combination, by weight: 0.60-0.67% carbon (C), 0.10-0.20% manganese (Mn), 0.10-0.20% silicon (Si), no more than 0.10% chromium (Cr), no more than 0.10% nickel (Ni), no more than 0.10% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), no more than 0.40% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
62ΠH1 has the following chemical composition, in combination, by weight: 0.60-0.67% carbon (C), no more than 0.06% manganese (Mn), no more than 0.06% silicon (Si), no more than 0.06% chromium (Cr), no more than 0.06% nickel (Ni), no more than 0.06% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), 0.20-0.30% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
80ΠΠ1 has the following chemical composition, in combination, by weight: 0.78-0.85% carbon (C), no more than 0.10% manganese (Mn), no more than 0.05% silicon (Si), no more than 0.10% chromium (Cr), no more than 0.10% nickel (Ni), no more than 0.10% copper (Cu), 0.03-0.10% aluminum (Al), 0.06-0.12% titanium (Ti), no more than 0.40% vanadium (V), no more than 0.040% sulfur (S), and no more than 0.035% phosphorous (P), with the balance being essentially iron (Fe).
An example of a process used to achieve desired material characteristics for high carbon content steels (0.60-0.80% carbon) such as 62ΠΠ1, 62ΠΠ2, 62ΠΠ3, 62ΠΠ4, 62ΠH1, and 80ΠΠ1, is thru-surface hardening (TSH). This process heats the steel in a controlled furnace atmosphere for about 40 minutes to one hour, and then subsequently quenches the steel in a water based solution. This process provides an irregular case profile and has a root case depth of approximately 0.045 to 0.060 inches for gears. The gear pitch line core hardness is greater than 55 Rockwell C and surface hardness is 58-63 Rockwell C. Microstructure 0.010 inches beneath the surface is martensite only for 0.60% carbon steel, and is martensite and retained austenite for 0.80% carbon steel.
Thus, high carbon content steels such as 62ΠΠ1, 62ΠΠ2, 62ΠΠ3, 62ΠΠ4, 62ΠH1, and 80ΠΠ1, do not require a lengthy carburization process to achieve desired material characteristics and instead can use a much shorter TSH process. However, TSH also has some disadvantages. The high carbon content makes machining very difficult. The core hardness is greater than 55 Rockwell C, which makes the gear teeth more brittle and more easily broken by shock loading. Further, when the microstructure consists mostly of martensite at the surface, wear resistance is adversely affected.
It is desirable to have an improved material that can be used to make driveline components, such as gears and shafts, that does not require prolonged carburization or thru-surface hardening, is less expensive, and has improved surface characteristics, as well as overcoming the other above-mentioned deficiencies in the prior art.